Competency based questions are not designed to catch you out, the interviewer just wants a better idea of your strengths in line with what they’re looking for.
Keep in your mind, or have notes in front of you, of the skills that the job description or company had listed as desirable. These are the “competencies” they’re most likely to ask you about. They’ll ask you to describe a situation you’ve been in – where you’ve had to lead a team, where you’ve dealt with a difficult situation, where you’ve had to be organised, etc.
This is your opportunity to draw on past experience, whether this was in the classroom or out of it, and highlight your strengths with clear and factual examples.
You should avoid giving vague answers, but really describe any problems that came up, how you dealt with the situation, and what you learned from it.
So instead of saying “I often lead group projects in school”, you could say “I used the communication skills I developed within my friendship group to lead and coordinate a difficult group project. I delegated a small part of the project to each member, kept in regular contact and updated the rest of the group. I acted as the central person to communicate with so that I’d have a clear understanding of each persons’ progress. I did this while managing my own time and efforts for my part of the project. This exercise really helped me learn how to coordinate separate pieces of work and use effective communication to meet deadlines and work as a team.”